Short answers, pulled from the story.
Insects are hexapod invertebrates with a chitinous exoskeleton supporting their three-part body plan of head, thorax, and abdomen. They possess three pairs of jointed legs attached to the thorax and a pair of antennae for sensing their environment.
Flying insects appeared much later during the mid-Carboniferous period around 328 to 324 million years ago. Claims about origins dating back to the Silurian or Devonian some 400 million years ago based on molecular clock estimates are unlikely correct given the fossil record.
Air enters through paired openings called spiracles located along the sides of the thorax and abdomen. These openings connect to internal tubes known as tracheae that deliver oxygen directly to tissues.
Hemimetabolous insects change gradually after hatching through stages called instars until reaching the final adult stage. Holometabolism or complete metamorphosis occurs where the insect changes in four distinct stages: egg, larva, pupa, and adult.
Insects account for vast majority of insect consumption yet also provide critical pest control services. The value of insect pollination of crops and fruit trees was estimated in 2021 to be approximately $34 billion in the US alone.